Conventionally, a xenon lamp having an electrical discharge space in which xenon gas is enclosed, is used widely for a light source apparatus which projects an image in a movie theater, etc. The light source apparatus is very large, so that a large installation area may be required therefore. However, with recent developments of the digital technology, a technology in which, instead of an image which was conventionally obtained through a film, a digital image is used has been developed. With such developments, a type of light source apparatus in which a digital image formed on a liquid crystal display or a digital mirror device (DMD: Registered trademark of TEXAS INSTRUMENTS, INC.), is projected and enlarged, starts to be used, and the light source apparatus itself is required to be small. Moreover, the xenon lamp arranged in the light source apparatus is also required to be small, compared with the conventional xenon lamp, so that the development of miniaturization has been advanced.
On the other hand, when projecting a digital image on a screen, as represented in the DLP (registered trademark) technology, a high quality image is required, and a high stability of light emitted from a xenon lamp arranged in the light source apparatus is also required. If operating time of the xenon lamp becomes long, flickering tends to occur. The reason is considered as set forth below. When switching on the xenon lamp, direct current is impressed therein. For this reason, electrodes, that is, a cathode and an anode are separately designed from each other. In general, the anode is large in volume, and is made of high melting point metal material, for example, pure tungsten metal material. Moreover, the cathode is small in volume, and is made of high melting point metal material containing the emitter (the so-called emitter substance). For example, an electrode made of a thorium tungstate (hereinafter referred to as Th-W) in which thorium is contained in tungsten, is used. Thermoelectrons are emitted from the cathode made from the Th-W electrode, at time of lamp lighting, so that electric discharge is maintained. Although electrons in the metal material are generally emitted as thermoelectrons by heating the metal material in a discharge lamp, when the electrode contains an emitter substance, it is possible to easily emit thermoelectrons therefrom without heating of the metal material.
Here, when the cathode contains thorium, since electron emission is carried out easily, the thermionic emission in low energy becomes possible. In an early stage of lamp lighting, the thorium sufficiently exists across the tip of the cathode, so that thermoelectron radiation is easily carried out even in low energy so that electric discharge of the lamp is stable. However, when the lamp is turned on for a long time, since the thorium is gradually evaporated etc. from a surface portion of the tip of the cathode, the quantity of the thorium contained at the tip of the cathode decreases, so that the thermionic emission from the tip of the cathode becomes difficult. In this case, the temperature of the cathode is raised by shrinking an arc of the cathode and increasing electric input per unit area, the electric discharge is maintained by emitting thermoelectrons without the thorium. Since the thorium which exists inside the cathode leaks therefrom to the cathode surface by raising the temperature of the cathode at this time, thermoelectrons can be emitted easily again. Then, the cathode maintains the electric discharge, while an arc is expanded again. The thorium which has leaked out to the cathode surface evaporates again, so that it goes into a thorium drain state because the electric discharge continues, and the arc is shrunk in order that the temperature of the cathode is raised as mentioned above. While repeating expansion and contraction of the arc, portions where thermoelectrons tend to be emitted at time of contraction are not fixed, and arc generating positions move. It is considered that the movement of the arc generating section and the expansion and contraction of the arc is the cause of generation of flickering of the arc.
Although flickering tends to occur in a xenon lamp as mentioned above if an operating time becomes long, it is necessary to stabilize light emitted from a xenon lamp when the xenon lamp is used as a light source apparatus, such as DLP (registered trademark) etc. For this reason, the various attempts have been made in order that an electric discharge arc of such a xenon lamp is stabilized. For example, Japanese Laid Open Patent Nos. 2003-51286 and 2004-95375 are known as such attempts. Japanese Laid Open Patent No. 2003-51286 teaches that flickering of an arc is controlled by line of magnetic force which is generated by using a movable magnet arranged in a direction perpendicular to a lamp axis connecting between electrodes which are arranged in a discharge lamp and which face each other.
Moreover, Japanese Laid Open Patent No. 2004-95375 teaches the technology of stabilizing an arc by providing at least three lead wires arranged in parallel to a lamp axis connecting electrodes of the discharge lamp and in point symmetry with respect to the lamp axis so that current is made to flow therethrough so as to form a magnetic field. In the technology disclosed in Japanese Laid Open Patent No. 2004-95375, an arc is compressed toward the central axis of the lamp by the magnetic field generated by current which flows through the lead wires arranged in point symmetry, so that the brightness density of the arc is increased and fluctuation of the arc is controlled.